Method for fabricating nylon bushings



15, 1950 L. L. STOTT 2,518,504

METHOD FOR FABRICATING NYLON BUSHINGS Filed June 17, 1947 2 Sheets-Sheet 1 INVENTOR w x M ATTORN EYS METHOD Filed June 17,. 1947 L. L. STOTT FOR FABRICATING NYLON BUSHINGS 2 SheetsSheef 2 INVENTOR d/"dz ATTORNEYS Patented Aug. 15, 1950 METHOD FOR FABRICATIN G NYLON BUSHINGS Louis L. Stott, Reading, P assignor to The P013!- mer Corporation, Reading, Pa., a corporation of Pennsylvania Application June 17, 1947, Serial No. 755,085

8 Claims.

7 This invention has to do'with bushings made of plastic materials, and, more particularly, has to do with the fabrication of bushings from thermoplastic materials which are subject to oxidation or degradation even at temperatures at about their melting or softening points, and/or which undergo a largevolumetric contraction upon' solidification. Most of such thermoplastic materials are also characterized by relatively low thermal conductivity, and because of various of these characteristics it has heretofore been difficult to form cast or molded bushings from them,

especially where the bushings are of substantial thickness. 7 i The linear polyamides are typical of materials jof the class mentioned,-a specific example being fpolyhexamethylene, adipamide, known in' the trade as nylon.

---While various of the characteristics of the polyamides give rise to diflicult problems in the fabrication of bushings, at the same time certain other propertiesthereof are of particular advantage in bushings, for instance, for hearing purposes, especially where lubrication problems are difl'icult. i i

This invention is of particular advantage in the fabrication of bushings from polyamide materials, and is therefore described herebelow as applied to such materials.

. Although bushings of relatively small outside diameter may be formed by injection molding, this expedient is not normally suitable in the case of. bushings of relatively great length, outside diameter, or having a relatively thick wall, for example, a bushing 8" long, outside diameter, and wall thickness of /2"- i In addition, as a result of the characteristic contraction of the polyamides upon solidification, static casting of bushings in a cylindrical mold having a central. core is impracticablebe- ,cause the contraction upon cooling is so great that the bushings shrink on the central core with a force so great as to prevent their removal, and, in some cases, as to destroy themselves by cracking, this difficulty bein especially troublesome where the bushing has a relatively thick wall.

. As a primary object, the invention has. in view improved methods for overcoming all of the above-mentioned difiiculties involved in the formation of bushings from the synthetic. linear polyamide materials.-

, According to-the invention, a. mold is provided whichis. not only adapted to be charged with the material to be moldedin solid, granular, or powdered'form and to contain the material durg 2 ing the melting process, but which is also adapted, after melting of the material, to be inserted in a centrifugal, castingmachine.

Another object of the invention is the protection of the material from oxidation during the whole time the material is at an elevated temperature. a a

The invention also contemplates a method for melting synthetic linear polyamide materials which provides for uniform and rapid penetration of heat to all portions, of the material, even where a relatively large mass is being handled.

A further object of the invention is a method for cooling the. contents of a bushing mold having a central core under such conditions as to prevent cracking of the bushing upon contraction. l l

How the foregoing and other objects are attained will be moreclearly understood from the description following herebelow and the drawings, in which: i

Figure 1 is an axial sectional view of one form of a bushing mold containing a charge of cold molding material ingranularor flake form;

Figure 2 is a view of the mold of Figure ,1 immersed in a heatingbath, the material in the moldbeing here shown as melted;

Figure 3 is a sectional view of the mold of Figure 1 mounted in a centrifugal casting machine and showing the condition and position of the solidified material; and

] Figure 4 is a sectional view of a modification of the mold mounted in a centrifugal casting machine and showing the position of the molten material just after the initiation of the spinning operation and before any solidification has occurred. i

As has been suggested above, the characteristics of synthetic linear polyamide materials and notably their sensitivity to degradation and oxidation at or near their melting points, renderit desirable that the exposure of the molten material to the atmosphere ,beikept to a minimum. I accomplish this by melting the material in the same vessel in which it is to be solidified, this vessel comprising at one and the same time a mold adapted to determine at least some of the dimensions of the finished bushing, to provide for transfer .of heat to the molding material during the melting operation, and to protect the material from contact with the atmosphere throughout the period during which it is at anelevated temperature. The mold, therefore, is' adapted for use both in thespecialized heatingprocess and the specializedcooling process of thepresent invention, details of which will appear more fully hereinbelow.

The tendency of synthetic linear polyamide materials to degradation during the melting operation increases with the amount of material being melted, since, the, low thermal conductivity of the material requiresa high temperaturev gradient to effect transfer of heat to the interior of the mass Within a reasonable length of time. If.

the time is short or the mass large, or both, this" temperature gradient will resultv raiSing. h external portions of the mass to the degradation point before raising the center to the melting point. By incorporating in themold a central' core adapted to the rapid transfer of heat to the surrounding material in the mold, I have proser ated: and the: whole mold placed in a heating hath containing a heating medium I1, as illustrated in Figure 2. As the temperature of the a mold; rises, the plugs I2 and I3 expand to a vided for application of heat bothexternally and. internally of the mass of material inthe, mold In this way, since the; heat is applied from two directions, the ternperature gradient is substanjtiially reduced 'and, therefore, a relatively much larger mass of materiar may berapidly heated, notwithstanding the low; thermak conductivity thereof and because of the reduced thermal gradient, degradationis avoided. Furthermore; by 'reducing'the maximum temperature at the outside of" the mass, the tendency to oxidation isalso l'substantially reduced. Sincethematerial is c 0mplet 'ely; enclosed during the entire elevated; telfnperature phase of the operation; contact with the "atmosphereis minimized, and the combination fthesefactors greatly reducesanytendency of themateriartooxldize. V j V .11 In t'h'e' form of the invention-illustratedm Fig;- ure- 'l, I employ a m,old having ahollow core, -preferably openat both ends to permit circulation therein 0f the, heating medium, As the wall 'iof this hollow core neednot be t-hielq it may; be

made fga variety of materials so long asit has reasonably good thermal conductivity; Steel tubing will serve the purpose, although for a ;hig h,er" rate of heat transfe1j, bronze orcopper maybe used; v r

In the form of 'the.i nyention illustrated inEig- 1 1 3214; jemplqy, a li ore. o material" havin high thermal conduetivity, for instance bronze h onp r t eh transfer i this case taki p acehr e nduqtmn a ly t o gh h ta qrithe c re rom. the e d ther o V I eliminate e p obl m. of: shr nkage-io h central core during solidification Of 'the material .hr pla ing he mo d; con aining the molten; a- -terial ina centriiugal casting machine and spin- Ythemold duringlthe solidification phase of th n p essl as res lt of which t e mate al is "maintained against the outside wall of'the mold. Since the cooling of the, material is accompanied Thy. a. eQhcu nt vol m on raction; he material; progressive y shrinks .cljear ofj'the central .core. g

Consider n now. the ventionv a ustrate ,in thedrayvingS Figure ljshows one form, of mold icompri sing an externalsleeve HI of heat resisting metal f'or .exampl'e steel', a h llow core H, a bottom plug 12*, and a top plug t3; both of which are forme Qfamaterial, such. as bron e, having a higher coefiicient of thermal expansion than that comprising sleeve. I ll. Bottom plug i2 is machined: tqrrqv de a. nu fit n sleeve II! w n cold] 019 pl s lt qul'dfi u ly w e c d- BQottom plug I2 is cut out; as at; M, and top plug L, l3 is .cut out; as at [5, to, engage; with and center sore H w l The inside diameter of-sleeye ll) and'the inside distance between plugs I"! and I3 determine the greater extent, than sleeve I0, thus insuring a liquid-tight fit. Heat transfer to the material is effected: not. only through sleeve [0 but also through core ll. As illustrated in Figure the hollow core I I permitscirculation of the heating medium l1 therethrough, thus increasing the rate-of'heat transferto the-central portion o f-the mold.

Aftenthe material is completely melted; the mold is placed in a centrifugal casting machine such-as isdisclosed in Figure-3 and'securedtothe spinningplate 29- b-y-studs l9, nuts 20; retainer bar- 2 and takeupbolt' 22". The take up bolt bears-against a plug' member 23 having= a flange z l which is adapted to engage with the end of core H. The moldis thus retained in position as against axial movement by-takeup bolti l; member 2k and studs l9; and against radial motion by-the side wali of circular recess 28 in spinner plate 29 of the centrifugal casting machine;

After the 'mold is secured in the centrifugal casting-maehine- (withthemolten materiat still in the condition indicated at- Ilia Figure it is rotated at a high speed, say from 2000 to 4600 R M1, whereupon the action of centrifugal force distributes the molten material axially of the'moldin -the form of acylinderin contactwith theoutsidewall (as indicated'at l fic in. the modificationof- Figure 4'); Upon cooling; the material contracts' andthusdraws further away from the central core, assuming on solidification" a position: and condi-tion indicated" atl=6bin Figure 3. i It be-noted that theretaining; force exerted by takeup bolt 2-2 impinges directly upon sleeve and is transmitted axially to spinner-plate-IB. Top plug t3 is-free of the force; imposed-bythe fastening means; and; therefore the axial digrnension of the bushing; is not influenced by; the

operation ofmountin the mold in.the casting machine, as would be the case if bolt 22bore directly against unsupported topplug l3 7 In Figure 4 I have, illustrated an, alternative form of moldin which is employed a solid central can: like. a er al ha ing highe thermal n.- ducti-vityhe matwialhflnsmolde is in, the, form illustrated in Fi ure 3., this cor penerangement also provides for fixing the axial dimension of the moldin a manner-which wili be Iiiiiaffectedbyinounti ng the mold in the spinn n a ines will be seernthe central core II or Ila not only provides for the transfer of heat to the interior of tlieinass but-also provides for the ,transmission of the axial thrust incident to maintaining the mold against the spinner plate.

As has been pointed out in my copending application, Serial Number 595,325, filed May 23, 1945-,now abandoned, in the molding of ynthetic While a sub- I have found that there is a pronounced directional cooling effect toward spinner plate 29. of the centrifugal casting machine. In fact, I have discovered that in some cases the rate of heat transfer to the spinner plate may be higher than is desirable, in which case I provide a gasket 3| of asbestos or other insulating material between the bottom plug l2 and spinner plate 29. By properly selecting gasket 3| for thickness and thermal conductivity, I can control the axial cooling of the molded bushing within quite accurate limits. of course, the gasket is not employed during the heating operation.

Inasmuch as cooling takes place while the mold is being spun, except during the first few revolutions of the mold, the heat transfer from the material is substantially confined to the end plugs and the external sleeve, since the material is almost immediately thrown out of contact with the central core by centrifugal force.

The rate of rotation of the mold during cooling is influenced by a number of factors, including the dimensions of the bushing and the rate and direction of heat transfer from the molten material. In all cases the speed should be high enough so that the liquid material will be thrown clear of the central core. As is well known, the bore of a vertical centrifugal casting may be considered to be, to a first approximation, a parabola, the inclination of the walls from the vertical per unit of height bein a function of speed. With vertical castings, therefore, the speed should be great enough so that the parabola will clear the cylindrical core of the mold. With practicable operating speeds, the parabola will clear the core when casting bushings up to about 12" in length, provided the inside diameter is not too small. Bushings of greater length may advantageously be cast horizontally, in which case, as is well known, the bore is substantially cylindrical regardless of length. By horizontal centrifugal cooling, bushings at least 4 feet in length may be made. Whatever the inclination of the axis during spinning, therefore, the speed is selected so that the bushing will not cool in contact with the core, and as a result, the problem of shrinkage on the core, with the resultant difficulty of removal or destruction of the bushing, is completely avoided.

In the various figures, I have shown the position and condition of the material at four stages of the process: in Figure l, the granular molding material packed into the cold mold; in Figure 2, the position of the molten material under static conditions, showing the net result of the volumetric expansion upon heating and the infilling of the spaces between the granules; in Figure 4, I have shown the position of the molten material at the commencement of the spinning operation, but before contraction andsolidification have begun; andin Figure 3, the position of the solidified bushing, indicating the contraction which has taken place concurrently with the cooling of the material. l 1 I Attention is called to applicants copending continuation-impart application Serial No. 166,790,- filed June 8, 1950, which discloses subject matter in common with the present application. r 7

Iclaim: r 5

1. A method of fabricating cylindrical bushings of synthetic linear polyamide materials which comprises melting the material in a mold having a central core of smaller diameter than the bore of the finished bushing, and rotating the mold during cooling of the contents to produce a bushing having an internal diameter in excess of the core diameter.

2. The process steps in the fabrication of bushings of synthetic linear polyamide materials characterized by high volumetric shrinkage upon solidification and by relatively low thermal conductivity, which comprises introducing the material in particle form into a mold comprising a sleeve and a core therein, the inside diameter of the sleeve being substantially equal to the desired outside diameter of the bushing, and the outside diameter of the core being less than the desired inside diameter of the bushing, applying heat to the sleeve and core to melt the material and thereafter rotating the mold about the axis of the core during cooling thereof.

3. The process of fabricating bushings of synthetic linear polyamide materials characterized by high volumetric shrinkage upon solidification and by relatively low thermal conductivity, which comprises introducing the material in particle form into a mold comprising a sleeve and a core concentric therewith in an amount having a cold volume less than the volume of the mold cavity, the inside diameter of the sleeve being substantially equal to the desired outside diameter of the bushing, and the outsidediameter of the core being less than the desired inside diameter of the bushing, applying heat to the material through the core and thereafter cooling the material while rotating the mold to produce a bushing having an inside diameter greater than the diameter of the core.

4. The process steps in the fabrication of bushings from thermoplastic materials, which comprise introducing the material in particle form into a mold comprising a sleeve and a core concentric therewith, the inside diameter of the sleeve being substantially equal to the desired outside diameter of the bushing, and the outside diameter of the core being less than the desired inside diameter of the bushing, effecting transfer of heat to the material in the mold by immersing the mold in a heating bath with the core in heat exchange relation to the bath and rotating the mold about the axis of the core during cooling of the contents thereof.

5. A process in accordance with claim 4 in which the said core comprises a hollow cylinder adapted to be penetrated by the heating bath.

6. A process in accordance with claim 4 in which the said core comprises a solid cylinder having higher thermal conductivity than the material to be molded.

7. A method for forming bushings from a synthetic linear polyamide, which method comprises charging the polyamide in particle form into a mold having a core disposed axially of the bore :mfflzthe? hashing-. toi-lbeef-ormed, melting the poly- 1 mideby-transfer of --heat 'radia11yinwardly-from heeexternakmold wvalband. r-adia11y outwardly iefromithe coreandicoolingzthe molten pol-yamide while rotating the mold-aabout the axisof 'the rs-core. 7

$8. Thexprocessstepsdn thefabrication. of'bushingswofssynthetic Iineampolyamidmaterials charaa'cterized my high -wolumetric shrinkage upon solidification, by relatively low thermal conductivity, and by sensitivity" to oxidation iin-the iemol'ten' conditionmvlzmieh;stepsoomprise charging 1 ":mbidiidefinedzioyt-a eylindrieaiouter wall and-a eattcondubtingcore with -a--mass-of material of hanithez 'vo-lumeaofthe mold cavity, sealingthe moId tra1isfenringmeat to the materialathrough he typeede'scrib'edewhose solidified volume is less t thecore, andi-rotatingathemold aboutitheiaxlsipf the core during coolin ofthe" 'contehtsifthetefif.

LOUIS 1 st11orr.

REFERENCESIJCITED The. following references areinf r'ecofd file of i this patent:

UNITED: STATES-PATENTS Number Name Nolever Talylor ,Bec.;\11;5, 1'942 

1. A METHOD OF FABRICATING CYLINDRICAL BUSHINGS OF SYNTHETIC LINEAR POLYAMIDE MATERIALS WHICH COMPRISES METLTING THE MATERIAL IN A MOLD HAVING A CENTRAL CORE OF SMALLER DIAMETER THAN THE BORE OF THE FINISHED BUSHING, AND ROTATING THE MOLD DURING COOLING OF THE CONTENTS TO PRODUCE A BUSHING HAVING AN INTERNAL DIAMETER IN EXCESS OF THE CORE DIAMETER. 